Summary: Photosynthetic activities of a synchronously grown aerobic N-fixing unicellular cyanobacterium, sp. strain Miami BG 043511, were examined. Cells 1 h after the onset of synchronous growth exhibited an O evolution rate of 400-500 mUmol (mg Chl aL) h at saturating light intensity, while it decreased to 40-80 mUmol (mg Chl ) h in 12 h cells. Changes in the relative fluorescence intensity (excitation wavelength 430 nm, emission 685 nm) were not as large as the changes in photosynthetic O evolution activity. Photosynthetic electron transport activities of both photosystem II and photosystem I in 12 h cells were lower than those in 1 h cells. Photosystem II activity with 2,5-dimethyl--benzoquinone, and photosystem I activity with diaminodurene and methyl viologen in 12 h cells were about 64 and 46%, respectively, of those in 1 h cells. However, the reduction in photosynthetic electron transport activities was not sufficient to account for the reduction in photosynthetic O evolution activity in 12 h cells, which was about 14% of that in 1 h cells. Based on these observations, the reduction in photosynthetic O evolution activity in 12 h cells was ascribed to the combined effect of a reduction in both the photochemical and the biochemical steps of photosynthesis.


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